The following disclosure is based on German Patent Application No. 100 65 198.4, filed on Dec. 20, 2000, which is incorporated into this application by reference.
1. Field of the Invention
The invention relates to an optical integrator for an illumination device for a microlithographic projection exposure system, as well as an illumination device equipped with the optical integrator.
2. Description of the Related Art
The performance of projection exposure systems for the microlithographic production of semiconductor components is essentially determined by the imaging properties of the projection system. In addition, the image quality and the achievable wafer flow rate with a wafer stepper or wafer scanner are also essentially determined by the properties of the illumination device upstream of the projection system. This must be capable to provide as large a quantity of light as possible in the case of homogeneous distribution of intensity in a way that is precisely adjusted to the downstream optical system.
The adjustment to the subsequent system is also essentially determined by the telecentric alignment at the exit of the illumination device. A high degree of homogeneity of the illumination falling on the photo mask can be achieved by mixing the light coming from a light source in the illumination device with the aid of so-called optical integrators or light mixing elements. In addition to the optical integrators working with honeycomb condensers, those optical integrators with a rod, which consists of a material transparent for the light of a light source of the illumination device and, which is essentially penetrated by radiation in its longitudinal direction (z direction), have also gained acceptance. The rod is also described as a glass rod in the following, although it does not only consist of a glass-type material, for example synthetic quartz glass, but instead can also consist of a crystalline material, calcium fluoride for example. The rod has an entrance surface optically facing towards the light source of the illumination device, in which the light of the light source is incident, as well as an opposing exit surface, which can form an intermediate field plane of the illumination device.
As the cross-section form of the rod is intended to be adjusted to the form of the surface to be illuminated, the rod cross-section of the rods considered here is rectangular with an aspect ratio between width (or x direction) and height (or y direction) deviating from the value of 1. In the glass rod the light passing through is totally reflected many times on the lateral boundary surfaces, as in a kaleidoscope, which allows an almost perfect mixture of non-homogeneous parts of light to be achieved. Thus, the exit surface of the rod is reproduced as an almost uniform illuminated field on the photo mask. Illumination devices, which use that type of rectangular rod as an optical integrator, are disclosed for example in German Patent Applications DE 44 21 053, DE 195 20 563 or DE 199 12 464.
It has become known that the distribution of the light energy at the exit of such rod-shaped optical integrators exhibits an undesired asymmetry in the angular space. This asymmetry is described in the following as (energetic) ellipticity of the pupil and can be described for every image location or point (x, y) of the exit surface of the rod. The description of the ellipticity is based on the consideration that light energy is emitted via a specific angular distribution from every image location, i.e. not only in the z direction (equivalent to the longitudinal direction of the rod) but also with components diagonal to the z direction. Whereas the energy density for all directions should be identical within the angular distribution, in real systems a deviation from the symmetry i.e. a deformation of the distribution of energy is observed, in which the light intensity in areas at the distance from the x axis is lower than that in those areas that are just in the same distance from the y axis. This can lead to an undesired irregularity of the illumination at the wafer.
It is an object of the invention to provide an optical integrator of the type mentioned above, which in comparison with conventional optical integrators allows improved homogeneity of the illumination, in particular with regard to angle-dependent intensity inhomogeneities.
To resolve this problem, according to one formulation, the invention proposes an optical integrator for an illumination device for a microlithographic projection exposure system. The optical integrator includes:
a rod made of a material transparent for the light of a light source;
the rod having an entrance surface optically facing towards the light source and an opposing exit surface;
the rod further having an essentially rectangular cross-section having a width and a height perpendicular to the width, an aspect ratio between the width and the height of the cross section deviating from unity; and
the optical integrator having compensation means for compensating direction-dependent total reflection losses of the rod.
Another solution is an illumination device including such an optical integrator. Advantageous further embodiments are specified in the dependent claims. The verbatim of all claims is incorporated by reference into the subject matter of the description.
An optical integrator according to the invention is characterized in that compensation means are provided for the compensation of direction-dependent total reflection losses of the rod. This proposal is based on the knowledge that the reflection of light beams on the broad and narrow lateral surfaces of the glass rod is not total, but rather incomplete, even in the case of optimally prepared lateral surfaces. The cause of this can for example be a roughness of the reflecting surfaces, resulting in light beams no longer being situated locally in the angle range of the total reflection and part of the light intensity being uncoupled accordingly. It is also possible that in the area of the glass rod surfaces impurity atoms are embedded in the rod material, with the result that the refractive index at the edge of the rod does not correspond to that of the material in the interior. This can contribute to a partial uncoupling of the light. Absorption effects in the surface area can also reduce the intensity of the total reflected light.
As the glass rod in the case of the optical integrators considered here is rectangular due to its construction, the number of the reflections in the case of the passage of light on the lateral surfaces is perceptibly different. Light, the plane of reflection of which is essentially oriented parallel to the wider side of the rod and which is reflected on the narrow sides, is perceptibly less frequently reflected in the mean than light which is predominantly reflected in planes, which essentially run in the longitudinal direction of the rod parallel to or at an acute angle to the narrow sides of the rod, i.e. close to the y direction. Due to the incomplete total reflection in the case of real systems, intensity losses result at the rod exit, the amount of which is essentially dependent on the number of reflections during the passage of light, as well as on the degree of total reflection losses per reflection process. This results in higher losses in the angle range with large y values in the case of essentially identical surface properties of the wide and narrow sides than in the case of x values of the same magnitude. A compensation for these disadvantageous effects and, thus, a reduction of the ellipticity of the pupil, can be achieved with the aid of suitable compensation means, wherein the products, given by the number of reflections multiplied by the total reflection loss per reflection, in both the x direction and y direction are aligned with each other.
This can for example be achieved by targetedly reducing the specific total reflectance of the narrow lateral surfaces, at which fewer reflections occur, so that the total reflection losses per reflection increase, and an alignment is effected between the x and y direction. The specific total reflectance can for example be reduced by roughening the surface at the appropriate surface over a suitably long selected section in the z direction or in at least one partial area of a suitable size, by providing a reflectance reducing coating, flushing it with a gas, wetting it with a liquid and/or targetedly dosing or coating it with the refractive index altering substances. To this end at least one partial area of at least one rod lateral surface can have a suitable coating or a surface roughening, i.e. a targeted enlargement of the surface roughness relative to the adjacent smooth surface sections. At least one device can be provided to contact at least one partial area of one or more outer rod surfaces with a liquid or gaseous fluid that alters the reflectance in the contacted partial area, in particular reduces the total reflectivity. The device can for example be a gas flushing device or a wetting device. It is also possible, by taking suitable measures, to increase the number of reflections in the x direction running parallel to the wide rod side, wherein if necessary the specific total reflectance can remain unaltered at the lateral surfaces. A combination of these measures is also possible.
Via one or more of these measures it is possible to assign the rod at least one rectangular cross-section of a transparent compensation section of a suitable length in the z direction, which is formed in such a way that its total reflection losses in the direction of width (parallel to the x direction) are greater than those in the direction of height (parallel to the y direction) perpendicular to it. As this causes ratios to dominate in the compensation section, which are the exact inverse to those in the rod, with regard to the total reflection losses, a compensation of the energetic asymmetry is possible at the rod exit, wherein its amount can be influenced via the compensation section design.
Variants are preferred in which a compensation section is provided at the entrance area of the optical integrator, wherein the compensation section can preferably be arranged in the direction of the passage of light before the entrance surface of the rod. This results in a large rod length being available behind the compensation section, which allows a sufficiently thorough mixing of the light.
The compensation section can be formed in one piece with the glass rod, but is preferably achieved via a separate optical module. This renders it possible to modify already existing systems according to the invention without complicated interventions into the production and installation process of the glass rods. By interchanging compensation sections various compensation degrees can also be set, in order to adjust the integrator system on various illumination modus for example.
A preferred optical integrator is characterized in that at least one rod arrangement is arranged before the entrance surface of the rod, the rod arrangement comprising several small rods made of a material that is transparent for the light of the light source, wherein the small rods each preferably have a rectangular cross-section with an aspect ratio that is the inverse of the aspect ratio of the rod. Hereby an inverse aspect ratio is not necessarily defined by the inverse proportion of the aspect ratio but instead generally, so that the aspect ratio of the small rods is less than 1 if the aspect ratio of the rod is greater than 1 or vice versa. Via such a rod arrangement serving as a compensation section, in which the small rods are essentially arranged to advantageously fill the cross-section with regard to the entrance surface of the rod, the ellipticity explained above can be reduced to a tolerable degree or, if necessary, can be completely eliminated. By arranging a set of smaller rods before the glass rod, which are also rectangular, the long sides of which are however parallel to the short side of the rod, the difference in the number of reflections in the x direction and y direction is reduced. A change in the specific surface reflectance can be additionally determined via e.g. coating, surface roughening and/or wetting with a fluid, but is not required in principle.
A complete compensation of the total reflection losses is possible. Due to geometric considerations it would require, in the case of unaltered surface properties, a derivable large number of small rods and/or very thin rods and/or a large passage length of the compensation section. In practice, however, a partial compensation can suffice, so that compensation sections can be compactly produced and integrated into the installation environment of the illumination device with minimal effort. The compensation section, in particular the rod arrangement, preferably has a length 5% greater than that of the rod, wherein the length of the compensation section is preferably more than 10%, in particular between approx. 15% and 50% of the length of the rod. For expediency the number of small rods is greater than the aspect ratio of the rod cross-section and can, for example, be in the range between 3 and 15 and up to 20 rods. The height of the small rods can essentially be equivalent to that of the rod. From the technical production point of view it is advantageous if the small rods have identical cross-sections and lengths. However, in principle, deviating dimensions of the individual small rods are also possible. If necessary a better mixture of light can be achieved if the small rods have different dimensions. It is possible that the small rods are formed of small plates or foils, i.e. in particular of such transparent elements in which case the ratio between width and height can be more than 100 or even more than 1,000.
In embodiments in which, for example, a module is separately represented by a compensation section formed by a rod arrangement, this is expediently arranged in such a way that an insignificant space of, for example, less than one millimeter remains between the compensation section and the entrance surface of the rod. This prevents any contact with the sensitive surfaces of the glass rod and compensation section and thus possible damages to these surfaces.
In order to be able to use the largest possible parts of the rod cross-section for a passage of light, the rod arrangement expediently has a dense packing of small rods, wherein the small rods can be essentially arranged, however preferably without mutual contact, and separated from each other by lateral spacing, which can, for example, be in the range of less than 0.5 mm or 0.3 mm. A dense packing, in which the individual reflection surfaces of the small rods are still essentially free, can be guaranteed via suitable intermediately placed spacers, e.g. foils which expediently consist of a material by which the total reflection is not influenced in its respective support area at the adjacent small rods.
It usually suffices if the degree of reflectance of the surfaces facing towards each other between the small rods is greater than 50%, wherein at least 75% is particularly preferred. An almost complete total reflection is not necessary here. A dense packing with spacers simplifies the handling of the rod arrangement and guarantees a parallelism of the rods in the arrangement setting and in operation. At the same time the individual small rods are optically decoupled from each other in such that the total reflection is essentially maintained. By setting suitable degrees of reflectance between the small rods a desired degree of mixing can be set in the area of the rod arrangement.
These and other features result from the description and the drawings as well as from the claims, wherein each of the individual features can always be realized individually or together in the form of sub-combinations in an embodiment of the invention and in other fields and can represent advantageous, as well as protectable embodiments.